1. Field of the Invention
The present invention relates to a pattern formation method and a method of manufacturing a Levenson-type mask.
2. Description of the Background Art
In semiconductor devices such as semiconductor integrated circuits, a photolithography technique may be used to form electrodes or interconnections. In a photolithography step, an exposure step is performed to expose a resist in a prescribed shape through a photomask. The resist is formed, for example, of a photosensitive resin and formed into a prescribed shape by performing development after the exposure step.
The mask used in the exposure step has a light transmitting portion and a light shielding portion. The photomask includes a phase shift mask for changing a phase of light transmitted through the light transmitting portion to increase a resolution. The phase shift mask includes a Levenson-type mask and a half tone-type mask. These photomasks use the interference action of light to increase a resolution.
In the Levenson-type mask, when a resist, which is for example positive, arranged on a surface of a substrate is processed into a prescribed shape, a light transmitting portion is formed on one side of opposite sides of a part to allow light to pass through and a light transmitting portion is formed on the other side to cause phase reversal with respect to the phase of light of the light transmitting portion on one side. The Levenson-type mask is a photomask formed such that the phases of light on opposite sides of a part are reverse to each other thereby improving the resolution at the part.
In the half tone-type mask, when a resist arranged on a surface of a substrate is processed into a prescribed shape, a light transmitting portion is formed in one of a part or a surrounding part of the part and a light-shielding portion is formed in the other, which allows light to partially pass through it and additionally reverses the phase, thereby improving the resolution at the above-noted part.
Japanese Patent Laying-Open No. 2002-229181 discloses a Levenson-type mask for forming a light shielding portion for isolated pattern element formation and a plurality of light shielding portions for forming periodic pattern elements on a transparent substrate. This Levenson-type mask has a phase shift portion and a light transmission portion arranged on opposite sides of the light shielding portion for isolated pattern element formation. A phase shift portion and a light transmitting portion are arranged on opposite sides of the light shielding portion for forming periodic pattern elements. The remaining part of the transparent substrate is covered with a light shielding portion. It is disclosed that the width of the phase shift portion for isolated pattern element formation is made approximately equal to the width of the phase shift portion for forming periodic pattern elements.
Japanese Patent Laying-Open No. 2003-168640 discloses a method of manufacturing a semiconductor device in which the phases of a fine line pattern formed by a phase shift mask and of a shifter pattern adjacent thereto in a certain range are reverse to each other. Preferably, at least four shifter patterns are provided with a fine line pattern formed by a phase edge interposed at the middle, and the adjacent shifter patterns are arranged to have opposite phases.
In photolithography, a method of performing exposures multiple times is known to form a fine part on a surface of a substrate.
For example, Japanese Patent Laying-Open No. 11-283904 discloses an exposure method including: a high-resolution exposure in which a pattern of a part difficult in line-width control is transferred using a phase shift pattern; and a normal exposure in which a pattern of a part easier in line-width control is transferred to a photoresist layer without using a phase shift pattern while the part of the photoresist layer to which the pattern has already been transferred by the high-resolution exposure is protected by a light-shielding portion of the mask pattern.
Furthermore, U.S. Pat. No. 5,858,580 discloses use of two mask processes. The first mask is a phase shift mask and the second mask is a single phase structure mask. The single phase structure mask allows exposure in such a manner that the region of the phase shift is not erased. In the single phase structure mask, an exposure is performed so as to avoid formation at the undesired part in a region other than the part formed by the phase shift mask.
In addition, Japanese Patent Laying-Open No. 01-283925 discloses an exposure method of patterning densely in a first region and patterning more coarsely than the first region in a second region. The first region is exposed by a mask pattern having a phase shift pattern to cause phase reversal of exposure light and the second region is exposed by a mask pattern having a light transmitting region and a non-transmitting region.
Japanese Patent Laying-Open No. 2004-247606 discloses a method of manufacturing a semiconductor device, in which in forming a gate comprised of a gate electrode and a gate interconnection, only a gate electrode pattern is formed by a double exposure process using a binary mask or a half tone mask as a first mask and a Levenson-type mask as a second mask, and thereafter a gate interconnection pattern is formed by an exposure process using a binary mask or a half tone mask as a third mask.
As described above, a phase shift mask or multiple exposure allows formation of a pattern including a part having a small minimum dimension. However, for example in a semiconductor device, further miniaturization is likely to be sought, and forming a fine pattern with high dimensional accuracy is currently desired.
In a Levenson-type mask, a light-shielding film such as a chrome film is arranged on a surface of a transparent substrate. An opening portion is formed to transmit light. The opening portion mainly includes a same phase opening portion and a reverse phase opening portion. The same phase opening portion is a region through which light is transmitted without a phase change and is formed of a main surface of the transparent substrate. In the reverse phase opening portion, a concave portion is formed or a shifter is arranged in the transparent substrate. The light passing through the reverse phase opening portion has its phase reversed.
Some Levenson-type masks including a concave portion in a transparent substrate as a reverse phase opening portion have an undercut portion formed such that the concave portion extends to below the end portion of a light-shielding film. In this Levenson-type mask, when the reverse phase opening portions are adjacent to each other, the undercut portions are adjacent to each other. With increased miniaturization of semiconductor devices, the distance between the undercut portions is reduced. As a result, the contact area between the light-shielding film arranged on the surface of the transparent substrate and the transparent substrate is reduced, so that the light-shielding film may sometimes strip away.